The present invention relates to a method of producing metal powders, and particularly to a method of producing high-purity, ultra-fine metal powders. The invention is particularly applicable for producing silver powders of high-purity and ultra-finess, and therefore is described below with respect to this particular application.
There are many users for silver powders, including electronic processes, batteries, conductive coatings and shielding materials, conductive inks, water purification, catalyst manufacture and dental amalgams. Silver powders may be produced by most of the method currently used to manufacture metal powders in general, including chemical precipitation, physical atmoziation or milling, thermal decomposition and electrochemical deposition. However, many applications require silver powder in ultra-fine form, i.e., with an average particle size less than 150 nm. These latter applications include use: as a catalyst in the oxygen electrodes of fuel cells; in high-performance positive electrodes of Ag-Zn, Ag-H.sub.2 and Ag-Cd batteries; and in low-temperature heat exchangers, conductive glues and films, conductive and semi-conductive polymers, adjustable resistors, and dispersion-strengthened materials.
My USSR Inventor's Certificate No.267079 published Jul. 16, 1970, discloses a method of producing ultra-fine silver powder based on the "Raney" method of making a "skeleton" catalyst, but avoiding the labor-consuming and environmental-dangerous crushing and nulling operations. The method therein disclosed involves the following procedure: form a silver-aluminum alloy having a silver content of up to 50% by weight; roll the alloy into foil strips having a thickness of 0.5-5 mm; surface-clean the foil strips, in order to remove aluminum oxide and other impurities; heat-treat the foil strips at 550.degree. C. in a protective atmosphere (e.g., argon), and maintain this temperature for about 1.5 hours in order to form a homogenized supersaturated solid solution of silver in aluminum, followed by quenching in water to fix the structure; leach out the aluminum from the alloy in a 20-40% solution of KOH or NaOH at a temperature of 0-80.degree. C.; wash-out the powder; and dry the powder.
The above method results in a very porous silver agglomerate (porosity of approximately 85%) consisting of particles having an average size of 30-150 nm (0.03-0.15 microns), a spheroidal shape, and a crystal face on the surface. These agglomerates are very weak, and their cohesive strength between particles is extremely low, enabling the agglomerates to be easily crushed by milling to form very fine particles.
While the technique described in the above publication produced ultra-fine silver particles, this technique has been found to have two drawbacks: The amount of residual aluminum in the silver powder made by this technique is relatively high, being about 0.5-1%; and the heat-treatment operation, to be performed in a protective (e.g., argon) atmosphere, is very complicated and costly.